A(1) Field of the invention
The invention relates to a microwave method for the density-independent measurement of the relative moisture content of an object to be measured comprising the steps of:
(1) providing the object to be measured in a microwave applicator, which microwave applicator, without an object to be measured, has known mechanical and electrical properties;
(2) measuring the influence which the object to be measured exerts on the dielectric properties of the microwave applicator at the measuring frequency.
It is possible to determine the moisture content of goods by weighing the goods in a dry condition and in a moist condition. The absolute moisture content is then equal to the difference of the result of the two weighings. The relative moisture content .PSI. (in percent) is equal to: EQU .PSI.(%)=100m.sub.w /(m.sub.w +m.sub.d)
wherein m.sub.w is the mass of the quantity of water present and m.sub.d is the mass of the dry material. Such a weighing method in a continuous process, however, is extremely time-consuming and expensive so that other measuring methods have already been available for some time.
For example, it is known from Hasted (ref. D(1)) that the complex dielectric constant .epsilon. of the moist material can be determined by measuring the complex microwave impedance (attenuation and phase shift). On the basis of the specific water properties in the microwave range, this complex dielectric constant is a measure of the absolute quantity of water present. An instrument based on this method is described in the article by Kraszweski (reference D(2)). By a separate density measurement, for example by weighing, the relative moisture content .PSI. can then be determined. So the above-mentioned disadvantage applies to the measurement of relative moisture content .PSI.. It has also been suggested (reference D(3)) to measure an object at constant density and temperature in a given shape and derive therefrom a calibrating curve for the relative humidity. A disadvantage is that the object to be measured must always be available in that given shape. In a continuous measuring process (conveyor belt) this is disadvantageous because the height of the object to be measured (wheat, coffee, sand, tobacco) on the conveyor belt is not constant and thus introduces a measuring error in determining the relative moisture content.
A(2) Description of the prior art
A method of the kind mentioned in the opening paragraph for measuring the relative moisture content .PSI. of an object independently of the thickness and/or density is known from reference D(4). It is stated in this article that, if the object to be measured is homogeneous and the attenuation and the phase shift vary linearly with the weight of the water and the weight of the dry object to be measured, a formula can be derived for the relative moisture content which is independent of the density or the height. Besides the restrictions which are imposed upon the derivation of the formula the dependence on the height has partly remained as is demonstrated in the article on page 368 in FIGS. 6 and 7. It has been found that the errors made depend on the moisture content and are therefore very disturbing.